1. Field of the Invention
The present invention relates to controlling handover and Quality of Service (QoS) in a wireless broadband access system. More particularly, the present invention relates to a method and apparatus for more accurately compensating QoS in a target Base Station (BS) after a handover.
2. Description of the Related Art
In a wireless broadband access system, a Service Flow (SF), necessary for data transmission/reception, is generated during network entry of a Mobile Station (MS). Each SF has a QoS profile in accordance with QoS requirements, and, depending on the QoS profile, a Base Station (BS) allocates wireless resources to the MS through appropriate scheduling.
When an MS moves between a serving BS and a target BS, a handover takes place between the BSs. During the handover between the serving BS and the target BS, the MS may suffer from interruption of its transmission traffic. Because of the procedure necessary for handover signaling, the traffic transmission/reception between the MS and the BSs may be interrupted for anywhere from tens of milliseconds (msec) to hundreds of msec. In this case, for an SF belonging to a QoS class having a real-time requirement, its QoS requirements may not be satisfied during the handover.
To provide the intact SF, which was being maintained in the serving BS, in the target BS, the Worldwide Interoperability for Microwave Access (WiMAX) Forum standard delivers an SF profile (SF_Info) from the serving BS to the target BS through an R6_HO_Cnf message of an R6 interface. However, since the target BS considers only SF_Info from the serving BS regarding static QoS requirements, such as the QoS class type, an MS, which has been handed over to the target BS, has no way to identify a dissatisfaction in terms of QoS requirements due to the handover break time. So, every QoS class type may cause various different problems.
According to the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard, there are four QoS class types including Unsolicited Grant Service (UGS), Real-time Polling Service (rtPS), Non-real-time Polling Service (nrtPS), and Best Effort (BE). Each of these service types has different traffic characteristics and QoS requirements.
Because UGS traffic requires a fixed rate, a BS's scheduler should immediately serve this traffic. As for rtPS traffic corresponding to a real-time SF, it has minimum required throughput per time and is limited in data transmission delay. In contrast, nrtPS traffic corresponding to a non-real-time SF is not limited in data transmission delay and has a maximum sustained rate condition meaning that despite sufficient capacity, the maximum rate should be sustained. BE traffic has the lowest priority in QoS class, and has only the maximum sustained rate condition without the need to guarantee a minimum reserved rate.
In the case of UGS, if an MS receives insufficient grants, the MS cannot help but notify of the abnormality using a Sleep Indicator (SI) bit. However, since it is provided in the standard that in response thereto, it should undergo compensation of about 1% for each SI bit, the response is delayed until the MS reaches the desired grants.
In the case of rtPS, to guarantee the minimum reserved rate, more grants are required. However, the grants are not received during the handover break and a target BS cannot determine the duration of the handover break. Therefore, the target BS has no criteria for adequately compensating the MS. Accordingly, the target BS may excessively compensate the MS, or the compensation may be inadequate and thus fail to satisfy the minimum reserved rate.
In addition, in the case of BE or nrtPS having the maximum sustained rate condition, since the target BS cannot determine the duration of the handover break, it is difficult to make adequate compensation so that the traffic should not exceed the maximum sustained rate.